The Data That Threatened to Break Physics

What does a rational scientist do with an impossible result?

Antonio Ereditato insists that our interview be carried out through Skype with both cameras on. Just the other side of middle age,…By Ransom Stephens

Antonio Ereditato insists that our interview be carried out through Skype with both cameras on. Just the other side of middle age, his salt-and-pepper hair frames wide open eyes and a chiseled chin. He smiles easily and his gaze captures your attention like a spotlight. An Italian accent adds extra vowels to the end of his words.

We talk for 15 minutes before he agrees to an on-the-record interview. He tells me he has no desire to engage journalists who might subvert his words into a sensational, insincere story. The reason he agreed to Skype with me is because I am not a journalist, but a physicist and writer who spent 13 years in the trenches of experimental particle physics. And he has no tolerance for entering another debate about behavior rather than science. But finally, he says, “Okay. I’ve looked in your eyes. I trust you. Maybe that is my problem. Maybe I trust too easily, but I trust you.” He laughs and leans back in his chair with his arms out and open.

Ereditato is the former leader of the 160 physicists from 13 countries that compose the OPERA collaboration, whose goal is to study neutrino physics. It was first proposed in 2000, and Ereditato led it from 2008 to 2012. Then in late winter of 2011, the impossible seemed to happen. “The guy who is looking at the data calls me,” Ereditato tells me from my computer screen. “He says, ‘I see something strange.’ ” What he saw was evidence that neutrinos traveled through 454 miles of Earth’s crust, from Switzerland to Italy—which they are supposed to do—at such a high speed that they arrived 60.7 nanoseconds faster than light could travel that distance in outer space—which should have been impossible.

News of the data leaked. People outside the experiment started gossiping about a violation of relativity, a result that would rattle the foundation of physics.

Over the last century, Einstein’s observation that no massive object can travel faster than the speed of light in a vacuum, enshrined in his theory of special relativity, has become a keystone of how we understand the universe. If the OPERA measurement was correct, it would mark the first-ever violation of that theory: An atom bomb in the heart of our understanding of the universe.

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I ask Ereditato if he thought it must have been a mistake. “I don’t think it’s fair to say this,” Ereditato tells me. “If we say that, we bias our analysis. So when we got this indication that something was so astonishing, the first reaction was, well, let’s find why this is so.”

Wolfgang Pauli postulated the existence of neutrinos in 1930 to solve a simple problem. When nuclei undergo beta decay through the emission of an electron or a positron, the electron’s antimatter equivalent, something is missing. Either something invisible is emitted along with the electron or positron, or energy must disappear. Since no repeatable experiment of anything flying, falling, moving, colliding, decaying, or staying put had ever seen energy disappear, Pauli proposed the neutrino, an invisible particle with all the properties necessary to bring beta decay into accord with the first law of thermodynamics. By invisible, I mean that when neutrinos pass through matter they rarely leave a trace. So rarely that it took almost 30 years before an experiment (by Frederick Reines and Clyde Cowan) found physical evidence of them.

Today, neutrinos are an integral part of the Standard Model’s periodic table of particle physics. Here you’ll find the particles that make up matter listed in pairs separated into three categories: electron neutrinos are paired with electrons, muon neutrinos with muons, and tau neutrinos with, you guessed it, taus. Neutrinos can morph from one flavor into another. For example, an electron neutrino can oscillate into a muon neutrino, and a muon neutrino can flip into a tau neutrino. “Neutrino oscillations are the first indication of physics beyond the Standard Model,” Ereditato tells me. Laughing, he adds, “That’s the reason why I like neutrinos.”

Which brings us back to the OPERA experiment. When it was conceived, evidence for neutrino oscillations was plentiful but all of it came from disappearance experiments. That is, the evidence consisted of either electron or muon neutrinos disappearing. An appearance experiment was needed, and that was OPERA’s goal. The idea was for CERN, the European Organization for Nuclear Research in Geneva, to produce a beam of muon neutrinos aimed at a detector buried deep beneath Italy’s Gran Sasso Mountain range, 454 miles away. If any tau neutrinos were detected there, then neutrino oscillations were happening. Following the particle physics tradition of snazzy acronyms for experiments, “Oscillation Project with Emulsion-tRracking Apparatus” became OPERA.

Measuring the speed of neutrinos as they traveled from the CNGS (CERN Neutrinos to Gran Sasso) beam to the OPERA detector was not mentioned in the proposal. But in February of 2011, OPERA turned most of its focus to exactly that.

I think as any scientist, [I was] very, very, very skeptical from day one,” says Ereditato. “You make a check list: timing, receiver, GPS, transmitter from the receiver to the detector, ... you check everything.” Some options were checked immediately, while others required them to wait. The CERN beam, for example, could not be stopped. In the meantime, Ereditato drove his team hard. “You could not imagine how I was handling this business with my colleagues—check this, check that, do this, do that, do this, let’s cut the chain, do it again, do it again—we did this from spring to September 23rd!”

The team tried and tested every permutation of software, hardware, and theory that they could think of, and through every step, every bug they fixed, every increment of understanding they earned, the evidence for faster than light neutrinos stood as solid as the mountain above the experiment. Then, the inevitable happened: News of the data leaked. People outside the experiment started gossiping about a violation of relativity, a result that would rattle the foundation of physics like it hadn’t been rattled since 1900, when Max Planck discovered quantum physics. The rumors “spread at the speed of light,” Ereditato tells me.

“And then what do you do? Think about yourself taking the position of spokesperson. Do you say: No, no comment? And then everyone will blame you, all journalists: ‘Oh you hide it. We want to know what is happening. We are taxpayers giving support to you, we have the right to know!’ Or you make a claim.” In a sinister voice, he adds: “I discovered the superluminal neutrinos.”

In this case, it wasn’t just up to Ereditato. Large experimental collaborations like OPERA have bylaws for dealing with controversy, and voted to announce the results in public by a large majority of the collaboration. Just a few individuals voted against the announcement. “Which I respect very much. And they were right, eventually, kudos.”

OPERA announced its results on September 23rd, 2011 at a special seminar at CERN. The team did not state that it had observed a violation of relativity, and instead of using phrases like “evidence for” or “discovery of,” it called the data an “anomaly.” But that pivotal caveat was lost in the sensation of human interaction. While the conditional made it into The New York Times headline, “Tiny Neutrinos May Have Broken Cosmic Speed Limit,” it did not make an appearance in The Daily Telegraph (“CERN Scientists ‘Break the Speed of Light’ ”) or The Guardian (“Faster Than Light Particles Found, Claim Scientists”) or Scientific American (“Particles Found to Travel Faster Than Speed of Light.”)

The worst data are better than the best theory. If you look for reasonable results, you would never make a discovery, or at least you will never make an unexpected discovery.

The physics community, on the other hand, received the announcement skeptically, even cynically. No practicing professional physicist was willing to abandon special relativity any more than Wolfgang Pauli was willing to abandon conservation of energy in 1930. Still, what if? Since the confirmation of the core tenets of the Standard Model by the UA1 and UA2 experiments at CERN in 1983, every discovery in particle physics (except for neutrino oscillations) had just added another checkmark to that annoyingly venerable Standard Model. How could particle physicists resist the temptation to hope that something, anything, might open up the field during their lifetimes?

Even Ereditato dared to hope. “You come out in a science conference, a seminar, and you start saying, ‘Hey guys, I have something which I don’t understand. Please help us in understanding.’ ” He pauses for a second and nods more to himself than to me. “I think it was a good choice to be modest. And second, is that everybody was dreaming that we were right. Everybody.”

In one direction lay epic, ground-breaking physics—and in the other, potential embarrassment. Should OPERA have waited? How many more months could they have spent analyzing and reanalyzing the result? Leaning forward and pointing at me through the camera, Ereditato explains why a scientist can’t ignore a measurement just because it seems absurd. “You don’t kill this ... nature is talking to us, not through theories, but through experimental results. The worst data are better than the best theory. If you look for reasonable results, you would never make a discovery, or at least you will never make an unexpected discovery. You only make—this is a contradiction in terms—an expected discovery.”

One thing is certain: The announcement got OPERA the help they had hoped for. A few days afterward, with the operators of the CNGS beam, they started developing a new approach to the measurement. The original analysis had to use a statistical technique to determine the neutrino’s arrival time because the beam was spread out in space. The new approach was to generate neutrinos in tight bunches so that they would arrive at the detector together, making it much easier to determine their arrival time.

It took two months to reconfigure the neutrino beam, perform the experiment and analyze the results—unprecedented speed for an experiment of this complexity.

The faster-than-light measurement was still there. “Then I started to feel fear,” Ereditato says. “I said, ‘Oh my God.’ And not only me, many people who were very critical, in front of this result, they had no more argument.”

tools for the search: A scientist studies the OPERA detector at the Gran Sasso National Laboratory.Alberto Pizzoli/AFP/Getty

Particle physics experiments consist of complex, building-sized detectors and 1particle accelerators. Design and construction begin years before the first bit of data is acquired. By the time both the detector and collider are up and running, the experimentalists have developed analysis software to sift through the data and separate signals from backgrounds, the rare and exotic from the common and mundane, music from noise. They use a version of blind analysis, like the double-blind tests of biomedical research that requires a “hidden signal box.” Instead of testing their techniques on real data, they test them on simulated data created by replicating the response of the detector hardware to known processes. This way, when they “open the box,” their measurements shouldn’t be biased by any conscious or unconscious desire for discovery.

Yet OPERA’s faster-than-light neutrino data persisted. The next step would be to seek independent confirmation outside of OPERA itself, which is common practice. The Higgs, for example, was observed by both the ATLAS and CMS experiments. But there were no other experiments that could confirm or deny OPERA for at least several years. There was, however, another experiment at the base of Gran Sasso, called the Large Volume Detector (LVD), that could at least check OPERA’s timing system. The idea was to make sure the clocks of each experiment were synchronized by comparing the arrival times of cosmic ray muons in their respective detectors.

“This was really the killing experiment,” Ereditato tells me. Looking back through all five years of OPERA data, the teams found a period when OPERA’s timing was off by about 73 nanoseconds. Then another mistake was found with the timing circuit that affected the bunched beam experiment: The frequency of OPERA’s clock wasn’t locked to the timing of the bunches. The combination of the two problems accounted completely for the 60 nanosecond early arrival time of the CNGS muon neutrinos.

The source of the timing problem was traced to a fiber optic cable that carried GPS timing signals from the surface of Gran Sasso, down 8.3 kilometers to the OPERA detector. The cable presented two insidious challenges: First, if it was poorly connected, one would reasonably expect that the receiver wouldn’t respond at all. If the receiver sees the light, it should fire the starting pistol for the neutrinos racing to the detector. If the receiver doesn’t see the light, the starting pistol shouldn’t be fired—an obvious bug. But neither of these happened. Instead, it took this particular receiver 73 nanoseconds to acquire enough light energy to fire the starting pistol and trigger the electronics; in effect, the starting pistol was fired 73 nanoseconds after the neutrinos left the starting line back at CERN. “I would have expected that you either get a signal, or you don’t. But you don’t get a delayed signal,” says Ereditato. Second, the cable’s connector seems to have moved. “The cable was in a good situation maybe one week before we started data taking, and it was in a good position when we checked again everything,” Ereditato says. “The nasty thing is that in the middle, where we took data for the neutrino velocity, the cable was in a strange situation.”

With the mistake found and fixed, OPERA’s measurement of the neutrino velocity is now the most accurate in the world. And it is perfectly consistent with Einstein’s special theory of relativity. The faint hope for new physics that wasn’t predicted by the venerable Standard Model was dead. But the performance of the OPERA team in finding a single loose cable among the thousands of electrical channels of experimental equipment was remarkable. “I am proud,” Ereditato tells me. “I should say very frankly, I was always thinking the solution would have come from very strange effects. Second order effects, somewhere that nobody thought about. I never thought such a thing (the cable), never.” Nor did the collaboration overstate the data, or make claims that were unwarranted. In fact, they made no claims at all, and worked closely with other teams in their investigation.

Nevertheless, it seemed clear that someone, somewhere, had made a mistake. Maybe it was the person who attached the cable or designed the receiver, or someone else entirely. In March of 2012, as the dust settled, the OPERA collaboration held another vote, this time to determine whether the collaborators had confidence in the leaders of the experiment. Each member institution got one vote. The tally ended at 16 to 13 favoring no confidence with several abstentions, well short of the two-thirds majority required to impeach the leadership, but enough to send a strong message. Both the OPERA leader, Ereditato, and the experimental coordinator, Dario Autiero, resigned.

Ereditato’s resignation letter made it clear he was resigning for the benefit of his team: “... as a result of the enormous media interest, the OPERA Collaboration found itself under anomalous and in some respects irregular pressure ... External tensions do not take long to transfer to the inside of a social system comprising over 150 people ... leading to the potentially dangerous outcome of potentially losing sight of scientific objectives ... This is a risk too great to run. To avert it, the position of individuals must take a back seat.”

But did Ereditato do anything wrong? People make mistakes. I spent 13 years working on experiments at SLAC, Fermilab, Cornell, CERN, and even the long dead Superconducting Super Collider. Maybe I’m more forgiving than people who haven’t crawled around under counting houses, routing and connecting cables. Connectors fail for lots of reasons and with thousands of channels, it’s probably safe to say that there is no particle physics experiment that doesn’t have to deal with a bum connector or two. They’re usually easy to find, but not this time. Some said OPERA should have done still more tests, but by the time of the no-confidence vote, a whole year had passed. How many more months should they have spent? Should their allegiance to special relativity have compelled them to wait until they found the problem? No, then they would be playing into the absurd concept that scientists are bound to uphold a scientific creed.

Maybe the frustration exhibited by the OPERA collaboration’s no confidence vote testifies to how much scientists, especially experimentalists, want to find something new, something that hasn’t been predicted, and how angry they get when it slips away.

I ask Ereditato to reflect on the whole experience. “Society likes black and white,” he answers. But answers in science are not always so cleanly resolved. “We have to be careful because if we give the impression that science never says yes or no, says always maybe, then people say, ‘Well, then I should not trust science.’ It is very delicate how to give this message.” Most science journalists are not scientists. “They pretend to treat scientific information the same way as murder or kidnapping.” As for his own role? “I learned that we are all actors in this field.”

Today, Ereditato is the director of the University of Bern’s Laboratory for High Energy Physics, and continues to participate in a variety of neutrino experiments. The OPERA experiment, meanwhile, has different leaders and continues to hunt for neutrino oscillations and collect tau neutrinos. They have four so far.

Ransom Stephens is a physicist, novelist, technologist, and science writer. His first popular science book, The Left Brain Speaks, but the Right Brain Laughs: an irreverent (but accurate!) look at the neuroscience of talent & skill, innovation & discovery, and art & science is coming out in October, 2015.

This article was originally published in our “Error” issue in May, 2015.

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